The present invention is generally directed to a structure and process for the fabrication of thin film field effect transistors employed in matrix addressed liquid crystal displays. More particularly, the present invention is directed to the utilization of specific materials in the field effect transistor (FET) fabrication process and structure. Even more particularly, the present invention is directed to the solution of material compatibility problems between aluminum contacts on amorphous silicon and indium tin oxide as a transparent electrode material.
A liquid crystal display device typically comprises a pair of flat panels sealed at their outer edges and containing a quantity of liquid crystal material. These liquid crystal materials typically fall into two categories: dichroic dyes and a guest/host system or twisted nematic materials. The flat panels generally possess transparent electrode material disposed on their inner surfaces in predetermined patterns. One panel is often covered completely by a single transparent "ground plane" electrode. The opposite panel is configured with an array of transparent electrodes, referred to herein as "pixel" (picture element) electrodes. Thus, a typical cell in a liquid crystal display includes liquid crystal material disposed between a pixel electrode and a ground electrode forming, in effect, a capacitor like structure disposed between transparent front and back panels. In general, however, transparency is only required for one of the two panels and the electrodes disposed thereon.
In operation, the orientation of liquid crystal material is affected by voltages applied across the electrodes on either side of the liquid crystal material. Typically, voltage applied to the pixel electrode effects a change in the optical properties of the liquid crystal material. This optical change causes the display of information on the liquid crystal display (LCD) screen. In conventional digital watch displays and in newer LCD display screens used in some miniature television receivers, the visual effect is typically produced by variations in reflected light. However, the utilization of transparent front and back panels and transparent electrodes also permits the visual effects to be produced by transmissive effects. These transmissive effects may be facilitated by separately powered light sources for the display, including fluorescent light type devices. LCD display screens may also be employed to produce color images through the incorporation of color filter mosaics in registration with the pixel electrode array. Some of the structures may employ polarizing filters to either enhance or provide the desired visual effect.
Various electrical mechanisms are employed to sequentially turn on and off individual pixel elements in an LCD display. For example, metal oxide varistor devices have been employed for this purpose. However, the utilization of thin film semiconductor switch elements is most relevant herein. In particular, the switch element of the present invention comprises a thin film field effect transistor employing a layer of amorphous silicon. These devices are preferred in LCD devices because of their potentially small size, low power consumption, switching speeds, ease of fabrication, and compatibility with conventional LCD structures. However, fabrication processes for certain desired semiconductor switch element structures have been found to be incompatible with the employment of certain materials used in the transparent LCD electrodes. More particularly, it has been found that it is desirable to employ an aluminum layer as the source and drain electrodes of the FET fabricated using amorphous silicon since conventional electrode materials, such as molybdenum, do not bond as well to amorphous silicon and may be more difficult to pattern. Investigations by the present inventors have indicated that good source and drain contacts to intrinsic silicon are most reliably obtained with aluminum metallization. Unfortunately, attempts at fabricating an LCD array has revealed a materials compatibility problem between aluminum and the indium tin oxide pixel electrode. A deterioration in the indium tin oxide (ITO) pixel electrodes resulted when aluminum, ITO and etchants, resist developer, or other aqueous solutions were simultaneously in contact. The result is a "Swiss cheese" appearance of the indium tin oxide layer. Accordingly, the problem addressed by the present application is that of devising a process that allows the advantages of aluminum source drain contact material while avoiding material compatibility problems, in a simple way, employing as few masking steps as possible. The number of masking steps is desired to be low since, in general, the greater the process complexity, the lower is the reliability of the resultant device and the process yield.
There is a large amount of literature describing amorphous silicon field effect transistors. Some of the literature that describes aluminum source drain FETs also discusses device properties with mere suggestions for display purpose applications. Other literature that describes display applications typically does not specify source or drain material but indicates cross sections showing that similar source-drain/ITO material compatibility problems have been experienced. The problem with the processes involved in these devices and others that have been considered by the present inventors is that they require as many as eight masking steps and thus require an extremely clean processing environment to achieve high production yields for LCD devices. As display size and complexity increases, these yield problems become more significant.
Articles in this vein have included the following: "Application of Amorphous Silicon Field Effect Transistors and Integrated Circuits" by AJ Snell et al., Applied Physics, Volume 26, pages 83-86 (1981); "Amorphous Silicon - Silicon Nitride Thin Film Transistors", by MJ Powell, Applied Physics Letters, Volume 38, No. 10 (May 1981); "Silicon TFT's for Flat Panel Displays" by F. Morin and M. LeContellec, Hewlett Packard Journal; "Application of Amorphous Silicon Field Effect Transistors in Addressable Liquid Crystal Display Panels", by GJ Snell et al., Applied Physics, Volume 24, pages 357-362 (1981); "A TFT-Addressed Liquid Color Display" by M. Sugatr et al., Proceedings of the Third International Display Research Conference, SID and ITE, Paper No. 5.3 (October 1983) and "Amorphous-Silicon Thin-Film Metal-Oxide-Semiconductor Transistors" by H. Hayama and M. Matsumura, Applied Physics Letters, Volume 36, No. 9 (May 1980).